Spinal Muscular Atrophy (SMA)
Description
Spinal muscular atrophy (SMA) is a rare genetic disorder that affects muscle strength and movement. It‘s caused by a faulty gene that prevents the production of a protein called survival motor neuron (SMN) protein. This protein is crucial for the survival and function of motor neurons, the cells that control voluntary muscle movement. Without enough SMN protein, motor neurons progressively degenerate, leading to muscle weakness and atrophy.
Genes Involved
SMA is caused by mutations in the survival motor neuron 1 gene (SMN1). This gene provides instructions for making the SMN protein. The severity of SMA depends on the number of copies of a related gene, SMN2. Individuals with more copies of SMN2 tend to have milder forms of SMA.
Recognizing the Signs and Symptoms
Symptoms of SMA vary depending on the severity of the condition, ranging from mild to life-threatening. Common signs include:
- Muscle weakness: This is usually the first symptom, often appearing in infancy. It typically begins in the legs and spreads to the arms and trunk.
- Delayed motor milestones: Babies with SMA may be slow to achieve milestones like sitting, crawling, and walking.
- Breathing difficulties: As the muscles responsible for breathing weaken, individuals with SMA may experience difficulty with lung function.
- Scoliosis: This condition, involving the sideways curvature of the spine, can occur due to muscle weakness.
- Feeding difficulties: Problems with swallowing and sucking may occur due to weak facial muscles.
Causes
SMA is caused by mutations in the SMN1 gene. These mutations prevent the production of functional SMN protein. Individuals with SMA inherit a copy of the mutated SMN1 gene from each parent. The condition is not caused by anything a parent did or did not do during pregnancy.
Inheritance/recurrence risk
SMA is an autosomal recessive disorder. This means that both parents must carry the mutated gene for their child to inherit the condition. If both parents are carriers, there is a 25% chance with each pregnancy that their child will have SMA, a 50% chance that their child will be a carrier, and a 25% chance that their child will not have SMA or be a carrier.
If one parent has SMA, the chances of their child inheriting the condition depend on whether the other parent is a carrier.
